Set max-width on preview image so it doesn't cause scroll-bars.

### Preview on thin embed

####  Before
![preview-thin-before](https://cloud.githubusercontent.com/assets/96776/4832055/31db155e-5f99-11e4-92bc-870a10a0667b.png)
#### After
![preview-thin-after](https://cloud.githubusercontent.com/assets/96776/4832054/31da9796-5f99-11e4-8ef6-c59355a30a1e.png)